1. Field of the Invention
This invention relates to the preparation of slurries and is particularly concerned with preparing a coal slurry substantially depleted in mineral-rich particles.
2. Description of the Prior Art
Processes have been developed in the past and are currently being developed to convert coal or similar carbonaceous material into more valuable and directly usable products such as synthetic oil, synthetic gas and the like. In some of these processes, such as proposed coal liquefaction processes, it is desirable to transport the coal in slurry form through the processing equipment. Normally the slurry is formed by first crushing the coal and then dispersing the resultant particulate matter in a suitable carrier liquid, such as a coal-derived oil. Some type of recirculation system in which slurry is removed from and then returned to its preparation zone is normally utilized to prevent excessive particle settling in the carrier liquid.
The particles of coal dispersed in the carrier liquid normally vary in size from fine particles, less than 325 mesh on the U.S. Sieve Series Scale, to any desired top size. The particles also vary in density because of the uneven distribution of mineral matter throughout the raw coal. Some of the particles are mineral-rich and have a density substantially greater than those particles which are composed primarily of organic material. The mineral-rich particles are hard and tough and therefore may have a deleterious effect on the particular processing system through which the slurry is passed. They may, for example, plug small openings in pumps, pipes, reactors and other processing equipment; jam check valves in high pressure positive displacement pumps; accelerate erosion and wear of internal equipment surfaces; and, because of their inert nature, decrease reactor utilization.
To help alleviate the above-mentioned problems, it is desirable to remove the mineral-rich particles from the slurrry before it is fed to its desired place of use. Removal methods based on size, such as screening, are not effective because the mineral-rich particles vary within the same size range as the organic-rich articles. Thus, it has been normal practice in the past to allow the mineral-rich particles to remain in the slurry as it is processed.